Aircraft hybrid fuel system

ABSTRACT

An aircraft hybrid fuel system includes a main tank and a set of flexible bladders, the main tank and the set of flexible bladders defining a fuel containment space. The system further includes a set of pathways coupling the set of flexible bladders to the main tank. The set of pathways is constructed and arranged to vent gas out of the set of flexible bladders into the main tank while fuel from a fuel source is provided into the fuel containment space defined by the main tank and the set of flexible bladders. Along these lines, each flexible bladder can be provisioned with a fuel port to provide fuel, and a separate vent port to vent gas to the main tank.

BACKGROUND

A conventional fixed-wing unmanned aircraft (UA) is an aircraft whichflies without a pilot on board. Such a UA typically holds fuel withinits wings.

One conventional approach to holding fuel within UA wings (hereinafterreferred to as the “wet-wing approach”) involves storing fuel within theUA wings themselves, i.e., seams, rivet holes, joints, and otheropenings are sealed to enable the hollow cavities of the UA wings tostore fuel directly. Another conventional approach to holding fuelwithin UA wings (hereinafter referred to as the “rigid tank approach”)involves placing rigid (or hard) tanks into the UA wings and storing thefuel in the rigid tanks. Yet another conventional approach to holdingfuel within UA wings (hereinafter referred to as the “bladder approach”)involves placing fuel bladders into the UA wings and storing the fuel inthe fuel bladders.

SUMMARY

Unfortunately, there are deficiencies to the above-describedconventional approaches to holding fuel within UA wings. For example,the above-described conventional wet-wing approach is susceptible toleaks. That is, each sealed seam, rivet hole, joint, etc. in each UAwing is a potential point of failure. Accordingly, the conventionalwet-wing approach requires constant inspection, and high maintenance toseal leaks when leaks are discovered.

Additionally, in the conventional rigid tank approach, the rigid tankstypically do not extend into gaps between framing members, tight areas,corners, etc. Accordingly, the conventional rigid tank approach does notmake very efficient use of available UA wing cavity space. Also, as fuelis consumed and air enters the rigid tanks of a UA, humans controllingthe UA from the ground become more reluctant to put the UA throughhigh-G maneuvers (i.e., aerial maneuvers subjecting the UA to highgravitational forces) for fear of uncovering the fuel suction ports ofthe rigid tanks and then drawing air within the rigid tanks into thefuel lines. Rather, the UA operator may avoid putting the UA throughextreme orientations once the rigid tanks get low on fuel in order toprevent exposure of the fuel suction ports.

Furthermore, in the conventional bladder approach, the bladders must befully evacuated to remove excess air before refueling the bladders. Itis also more difficult to find pinhole leaks within fuel bladders. Oneconventional inspection and refueling procedure requires the use ofspecialized fuel evacuation equipment and requires that the bladders areable to hold a vacuum for five minutes. Accordingly, the conventionalbladder approach is expensive, burdensome, and time consuming.

In contrast to the above-identified conventional approaches to holdingfuel within a UA, improved techniques are directed to an aircraft hybridfuel system which includes a main tank and a set of flexible bladders,each of which vents into the main tank. With placement of flexiblebladders in the aircraft wings, fuel is able to be stored with lessleakage risk than that of the conventional wet-wing approach and moreefficiently than that of the conventional rigid tank approach.Additionally, fuel within the flexible bladders can be completely drawndown without concern over exposing suction ports of the flexiblebladders thus enabling full use of the bladder-stored fuel and freeingthe aircraft of maneuvering restrictions. Furthermore, since eachflexible bladder vents to the main tank, there is no need to evacuatethe flexible bladders to remove excess air during refueling. Rather, anygases within the flexible bladders simply vent to the main tank as theflexible bladders are filled with fuel. Moreover, both the main tank andthe set of flexible bladders can be fueled to a positive pressure thusmaximizing the fuel storage capacity of the hybrid fuel system.

One embodiment is directed to a method of operating a hybrid fuel systemof an aircraft. The method includes preparing the hybrid fuel system ofthe aircraft to receive fuel from a fuel source, the hybrid fuel systemincluding a set of flexible bladders and a main tank. The method furtherincludes providing, after the hybrid fuel system of the aircraft isprepared to receive fuel from a fuel source, fuel from the fuel sourceinto a fuel containment space defined by the set of flexible bladdersand the main tank while venting gas out of the set of flexible bladdersinto the main tank. The method further includes preparing, after fuelfrom the fuel source is provided into the fuel containment space, thehybrid fuel system to deliver the fuel to an engine of the aircraft.

In some arrangements, each flexible bladder includes a venting end and asealed end. In these arrangements, providing fuel from the fuel sourceinto the fuel containment space while venting gas out of the set offlexible bladders includes:

-   -   (i) orienting the aircraft at an angle in which the venting end        of each flexible bladder is positioned above the sealed end of        that flexible bladder, and    -   (ii) after orienting the aircraft at the angle, supplying fuel        to fill the sealed end of each flexible bladder prior to filling        the venting end of that flexible bladder.

In some arrangements, each flexible bladder includes (i) a containerportion, (ii) a fuel port coupled to the container portion, and (iii) aventing port coupled to the container portion. In these arrangements,supplying fuel includes providing fuel through the fuel port of eachflexible bladder while gas within that flexible bladder exits thatflexible bladder through the venting port of that flexible bladder.

In some arrangements, the venting port of each flexible bladder connectsto the main tank through a respective venting tube. In thesearrangements, providing fuel includes supplying fuel through the fuelport of each flexible bladder while gas within that flexible bladderexits that flexible bladder into the main tank through the venting portof that flexible bladder and through the respective venting tubeconnecting that venting port to the main tank.

In some arrangements, a transfer pump is interconnected between the maintank and the set of flexible bladders. The transfer pump is constructedand arranged to pump fuel from the set of flexible bladders into themain tank. In these arrangements, supplying fuel through the fuel portof each flexible bladder while gas within that flexible bladder exitsthat flexible bladder into the main tank includes providing fuel intothe set of flexible bladders through a set of paths that circumvents thetransfer pump.

In some arrangements, a bypass valve is interconnected between the maintank and the set of flexible bladders. The bypass valve is constructedand arranged to allow and disallow fuel to flow between the set offlexible bladders into the main tank. In these arrangements, supplyingfuel through the fuel port of each flexible bladder while gas withinthat flexible bladder exits that flexible bladder into the main tankincludes:

-   -   (i) transitioning the bypass valve from a closed state to an        open state to allow fuel to flow from the main tank into the set        of flexible bladders, and    -   (ii) after the bypass valve is transitioned to the open state,        providing fuel from the fuel source into the main tank.

In some arrangements, transitioning the bypass valve from the closedstate to the open state includes forming a bypass valve path between themain tank and the set of flexible bladders. The bypass valve path runsparallel to a transfer pump path between the main tank and the set offlexible bladders. The transfer pump path is formed by the transferpump. In these arrangements, fuel in the main tank is allowed to flowfrom the main tank into the set of flexible bladders through the bypassvalve path when the bypass valve is in the open state.

In some arrangements, the main tank includes a hard tank. In thesearrangements, providing fuel from the fuel source includes filling thehard tank with fuel to sustain a positive fuel pressure within the hardtank (e.g., substantially 2.5 pounds per square inch).

In some arrangements, a positive pressure check valve is coupled to thehard tank. In these arrangements, providing fuel from the fuel sourcefurther includes stopping fuel delivery to the hard tank when fuelescapes from the hard tank through the positive pressure check valve(e.g., while fuel escapes at 2.5 psi).

In some arrangements, a negative pressure check valve is coupled to thehard tank. In these arrangements, the method further includes freeingthe negative pressure check valve from obstructions to enable air topass through the negative pressure check valve into the hard tank toprevent occurrence of negative pressure in the hard tank.

In some arrangements, the set of flexible bladders includes a left wingbladder disposed in a left wing of the aircraft and a right wing bladderdisposed in a right wing of the aircraft. In these arrangements,providing fuel from the fuel source includes expanding the left wingbladder and the right wing bladder with fuel from the fuel source. Theset of bladders is constructed and arranged to collapse as fuel ispumped from the set of bladders into the main tank.

In some arrangements, the method further includes operating a fueldelivery regulator to deliver fuel from the main tank to an engine ofthe aircraft and operating a transfer pump which transfers fuel from theset of flexible bladders to the main tank. Accordingly, the transferpump is able to move fuel from the set of flexible bladders into themain tank while the regulator delivers fuel from the main tank to theengine.

In some arrangements, the method includes discontinuing operation of thetransfer pump once fuel originally within the set of flexible bladdershas been transferred to the main tank while continuing to operate thefuel delivery regulator to continue delivery of fuel from the main tankto the engine of the aircraft. Accordingly, each flexible bladder can befully drawn of fuel thus maximizing bladder utilization.

In some arrangements, the aircraft is an unmanned aerial vehicle (UAV).In these arrangements, the fuel source can be a portable fuel tank andproviding fuel from the fuel source can involve supplying fuel from theportable fuel tank into the main tank through a fuel port of the UAV,the fuel filling the set of flexible bladders and venting gas from theset of flexible bladders into the main tank as the fuel is suppliedthrough the fuel port of the UAV.

Another embodiment is directed to an aircraft hybrid fuel system. Thesystem includes a main tank and a set of flexible bladders, the maintank and the set of flexible bladders defining a fuel containment space.The system further includes a set of pathways coupling the set offlexible bladders to the main tank. The set of pathways is constructedand arranged to vent gas out of the set of flexible bladders into themain tank while fuel from a fuel source is provided into the fuelcontainment space defined by the main tank and the set of flexiblebladders.

Other embodiments are directed to aerial vehicle apparatus, assemblies,subsystems, components, and so on. Some embodiments are directed tovarious processes and equipment which involve an aircraft hybrid fuelsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages will beapparent from the following description of particular embodiments of thepresent disclosure, as illustrated in the accompanying drawings in whichlike reference characters refer to the same parts throughout thedifferent views. The drawings are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of variousembodiments of the present disclosure.

FIG. 1 is a general diagram of an aircraft which includes a hybrid fuelsystem.

FIG. 2 is a general view showing particular details of the hybrid fuelsystem of FIG. 1.

FIG. 3 is another view of the hybrid fuel system during a re-fuelingprocess.

FIG. 4 is a flowchart of a procedure which is performed using the hybridfuel system of FIG. 1.

DETAILED DESCRIPTION

An improved technique is directed to an aircraft hybrid fuel systemwhich includes a main tank and a set of flexible bladders, each of whichvents into the main tank. With placement of flexible bladders in theaircraft wings, engine fuel can be stored with less leakage risk thanthat of conventional wet-wings and more efficiently than that ofconventional rigid tanks. Additionally, fuel within the flexiblebladders can be completely drawn down without concern over exposingsuction ports of the flexible bladders thus enabling full use of thebladder-stored fuel and freeing the aircraft of maneuveringrestrictions. Furthermore, since each flexible bladder vents to the maintank, there is no need to evacuate the flexible bladders to removeexcess air during refueling. Rather, any gas within the flexiblebladders simply vents to the main tank as the flexible bladders arefilled with fuel. Moreover, both the main tank and the set of flexiblebladders can be fueled to a positive pressure thus maximizing the fuelstorage capacity of the hybrid fuel system.

FIG. 1 shows a general top view of an aircraft 20 which utilizes ahybrid fuel system. The aircraft includes an airframe 22, an electronicflight controller 24, an engine 26, a hybrid fuel system 28, and apayload 30. Although these components are not shown coupled together, itshould be understood that these components are connected together, e.g.,via mechanical elements, electronic control lines, sensors, and so on.By way of example, the aircraft 20 is described below as a fixed-wingunmanned aerial vehicle (UAV).

The airframe 22 is constructed and arranged to provide support andprotection for the other components of the aircraft 20. As shown in FIG.1, the airframe 22 includes, among other things, a main body section 40,a left wing 42(L), a right wing 42(R), and a tail section 44.

The electronic flight controller 24 is constructed and arranged tocontrol the operation of the aircraft 20. Along these lines, theelectronic flight controller 24 is capable of executing a preprogrammedflight plan, operating in accordance with instructions from a groundcontrol station (GCS), or both. In particular, the electronic flightcontroller 24 is capable of controlling air speed via the engine 26,lift via angular deflection of ailerons of the left and right wings42(L), 42(R) (collectively, wings 42), direction via operation of thetail section 44, and so on.

The engine 26 is constructed and arranged to provide propulsion for theaircraft 22. To this end, the engine 26 operates under control from theelectronic flight controller 24.

The hybrid fuel system 28 is constructed and arranged to provide fuel tothe engine 26. As will be explained in further detail shortly, thehybrid fuel system 28 includes a main tank 50, a left flexible bladder52(L) disposed in the left wing 42(L), a right flexible bladder 52(R)disposed in the right wing 42(R), and a set of pathways 54 which enablethe flexible bladders 52(L), 52(R) (collectively, flexible bladders 52)to vent gases into the main tank 50 while receiving fuel. With suchventing of gas from the flexible bladders 52 into the main tank 50during refueling, there is no need to evacuate the flexible bladders 52prior to refueling as in a conventional bladder approach. Furthermore,fuel is stored efficiently within the wings 42 without leakage drawbacksof conventional wet-wings, and without fuel storage/usage inefficienciesof conventional rigid tanks.

The payload 30 is constructed and arranged to facilitate a particularmission or objective of the aircraft 20. For example, for areconnaissance mission, the payload 30 may include surveillanceequipment such as cameras, a radar subsystem, infrared sensors, etc. Foran attack mission, the payload 30 may include a set of bombs, missiles,guns, and/or other artillery. For communications, the payload 30 mayinclude wireless communications equipment, relays, and so on. In somearrangements, there may be multiple payloads 30 of different types thusenabling the aircraft 20 to perform a variety of operations duringflight.

It should be understood that the particular aircraft wing shape,dimensions, and scale may be different than that shown in FIG. 1. Forexample, it should be understood that each wing 42 of the aircraft 20includes ailerons, actuators, electronics, etc. and that the flexiblebladders 52 are able to expand around these components and into tightspaces to efficiently utilize space within the wings 42. Further detailswill now be provided with reference to FIG. 2.

FIG. 2 shows a side view of the aircraft hybrid fuel system 28 (also seeFIG. 1). As shown in FIG. 2, the hybrid fuel system 28 includes a maintank 50, a set of flexible bladders 52, a set of fuel pathways 54, atransfer pump 60, a manually controlled bypass valve 62, a positivepressure check valve 64, a negative pressure check valve 66, an in-tankpump/regulator 68, and a fill line 70. The flexible bladders 52 areshown slightly tilted inward (i.e., the outboard ends being higher inthe Z-direction than the inboard ends) to reflect possible anhedralaspects of the aircraft 20.

The main tank 50 includes a hard tank with stiff walls, and resideswithin the central body cavity of the airframe 22 (FIG. 1). Accordingly,the main tank 50 does not collapse in the manner of a flexible bladderas fuel is drawn from the main tank 50 for combustion by the engine 26.Rather, the main tank 50 is constructed and arranged to substantiallymaintain its shape regardless of the amount of fuel it holds.

Each flexible bladder 52 includes a vent port 80, and a fuel port 82with a perforated draw tube 84. In particular, the left flexible bladder52(L) includes a vent port 80(L), and a fuel port 82(L) with aperforated draw tube 84(L). Likewise, the right flexible bladder 52(R)includes a vent port 80(R), and a fuel port 82(R) with a perforated drawtube 84(R).

Each vent port 80 couples to the main tank 50 via a venting tube 90. Inparticular, the vent port 80(L) of the left flexible bladder 52(L)connects to the main tank 50 through a venting tube 90(L). Similarly,the vent port 80(R) of the right flexible bladder 52(R) connects to themain tank 50 through a venting tube 90(R). Such venting tubes 90 areconstructed and arranged to carry gases from the flexible bladders 52 tothe main tank 50 while the flexible bladders 52 fill with fuel.

Additionally, each fuel port 82 leads via a fuel line 92 to an input 94of the transfer pump 60. As shown in FIG. 2, the fuel line 92 includes abranch 93 in order to connect to both fuel ports 82 to the input 94 ofthe transfer pump 60.

An output 96 of the transport pump 60 couples to the main tank 50.Accordingly, when the transfer pump 60 is in operation, the transferpump 60 draws fuel from the flexible bladders 52 through their fuelports 82 into the main tank 50.

Furthermore, the bypass valve 62 is coupled between the fuel line 92 andthe main tank 50 to provide a parallel path to that of the transfer pump60. That is, the bypass valve 62, when in an open state, allows fuel tocircumvent the transfer pump 60 and flow between the main tank 40 andthe flexible bladders 52, e.g., from the main tank 50 into the flexiblebladders 52 through the bypass valve 62. Accordingly, as the flexiblebladders 52 fill with fuel, any gases within the flexible bladders 52are expressed from the flexible bladders 52 via the venting tubes 90into the main tank 50.

However, the bypass valve 62, when in a closed state, does not allowfuel to circumvent the transfer pump 60. As a result, when the transferpump 60 is in operation and the bypass valve 62 is closed, the transferpump 60 is able to reliably draw fuel from the flexible bladders 52 intothe main tank 50.

The positive pressure check valve 64 is constructed and arranged toprevent fuel under a predefined positive pressure from escaping a fuelcontainment space 98 formed by individual spaces within the main tank 50and the set of flexible bladders 52. Rather, fuel is able to fill thecontainment space 98 and remain positive pressure. Since the positivepressure check valve 64 points in the outward direction relative to themain tank 50, the fuel containment space 98 can be filled with fueluntil all gases are expressed through the positive pressure check valve64, the fuel is eventually expressed from the positive pressure checkvalve 64 and is sustained under positive pressure. Suitable values forthe predefine positive pressure include positive pressure values in therange of 1.0 to 5.0 psi (e.g., 1.5 psi, 2.0 psi, 2.5 psi, 3.0 psi, and3.5 psi).

The negative pressure check valve 66 is constructed and arranged toallow air to enter the main tank 50. Since the negative pressure checkvalve 66 points in the inward direction relative to the main tank 50, nonegative pressure is allowed to build within the fuel containment space98 thus ensuring that fuel flows in only one direction, i.e., to theengine 26 (FIG. 1).

The in-tank pump/regulator 68 is disposed within the main tank 50 andsupplies fuel from the main tank 50 to the engine 26 during operation.It should be understood that the in-tank pump/regulator 68 is shown atthe top of the main tank 50 for simplicity even through the in-tankpump/regulator 68 is typically located at the bottom (e.g., in a fuelgathering reservoir within the main tank 50).

The fill line 70 is constructed and arranged to carry fuel from anexternal fuel source to the main tank 50. In particular, the fill line70 leads from a fuel filling port from the outside of the aircraft 20 tothe main tank 50. The fuel filling port is accessible by a user (e.g.,by opening a lid, by removing a cap, etc.). Details of the operation ofthe hybrid fuel system 28 will now be provided with reference to FIG. 3.

FIG. 3 is a view of the hybrid fuel system 50 during refueling. Asshown, the flexible bladders 52 are now oriented so that the outboardends are lower than the inboard ends, and so that the vent ports 80 areat the top of the flexible bladders 52. It should be understood thatthere are a variety of ways to orient the flexible bladders 52 into thisgeometry. Along these lines, the aircraft 20 can be angled or tilted(e.g., pointed nose up, pointed nose down, etc.). In some arrangements,the aircraft 20 can even be placed substantially upside down to orientthe flexible bladders 52 into this geometry. In some arrangements, thewings 42 of the aircraft 20 are bendable (e.g., elastic) and aretemporarily flexed and held in these positions during refueling.

Once the flexible bladders 52 of the hybrid fuel system 28 are orientedin the manner shown in FIG. 3 where the venting ports 80 are at the topand the fuel ports 82 are lower than the venting ports 80, a user thentransitions the bypass valve 62 from the closed state to the open state.Accordingly, a pathway is now available which runs parallel to thetransfer pump 60 between the main tank 50 and the set of flexiblebladders 52.

An external fuel source 100 then provides fuel 102 into the main tank 50through the fill line 70 (see arrow 104 in FIG. 3). As the fuel 102enters the main tank 50, the fuel 102 flows through the bypass valve 62and through the fuel line 92 (see arrow 106), and into the flexiblebladders 52 through the fuel ports 82. As the flexible bladders 52 fillwith fuel 102, any gas within the flexible bladders 52 is displacedthrough the vent ports 80 and through the venting tubes 90 into the maintank 50 (see arrows 108). That is, as fuel 102 enters each flexiblebladder 52, the fuel 102 initially fills the sealed end (e.g., theoutboard end) before filling the vented end (e.g., the inboard end).Accordingly, any gas percolates to the tops of the flexible bladders 52and escapes through the vent ports 80. As a result, the flexiblebladders 52 do not need to be evacuated (as in a conventional bladderapproach) in order to purge the flexible bladders 52 of any gases.Rather, the flexible bladders 52 are simply filled with fuel 102 and gasis automatically vented through the vent ports 80 into the main tank 50.

During the course of filling the main tank 50 with fuel 102, the maintank 50 builds up positive pressure. When the positive pressure exceedsthe threshold of the positive pressure valve 64, gas first exits throughthe positive pressure valve 64 since the positive pressure valve 64 islocated at the top of the main tank 50. Moreover, the flexible bladders52 become completely filled with fuel 102 and fully expand into thecavities of the wings 42 (FIG. 1) to maximize utilization of wing space.

After all of the gas is forced from the flexible bladders 52 through theventing tubes 90, the vent ports 80 pass fuel 102 into the venting tubes90 which then enters the main tank 50. Ultimately, the entire fuelcontainment space 98 formed by the main tank 50 and the flexiblebladders 52 becomes filled with fuel 102 under positive pressure.Accordingly, fuel 102 exits through the positive pressure valve 64 whichsignals the user that the hybrid fuel system 28 is filled with fuel 102.The user then stops delivery of fuel 102 from the fuel source 100 anddisconnects the fuel source 100 from the fill line 70. Additionally, theuser closes the bypass valve 62 to prevent fuel from circumventing thetransfer pump 60. Furthermore, the user may cap (or seal) the fill line70 to prevent fuel from escaping through the fill line 70.

At this point, the hybrid fuel system 28 is maximally filled and readyto deliver fuel to the engine 26. In particular, during flight, theelectronic flight controller 24 (FIG. 1) activates the in-tankpump/regulator 68 to deliver fuel 102 from the main tank 50 to theengine 26, and the transfer pump 60 to draw fuel from the flexiblebladders 52 into the main tank 50.

As fuel is drawn by the transfer pump 60 into the main tank 50, theflexible bladders 52 are designed to collapse thus enabling full use ofall the fuel 102 in the flexible bladders 52. In some arrangements,sensors monitor operation of the transfer pump 60 during flight. Forexample, the revolutions per minute (rpms) of the transfer pump 60 canbe measured to determine when the transfer pump 60 is no longer pumpingfuel 102 (i.e., the transfer pump may spin at a fast rate when there isno fuel 102 in the flexible bladders 52 left to pump).

In some arrangements, the venting pathways between the vent ports 80 andthe main tank 50 (see the venting tubes 90) are provisioned with checkvalves that are oriented toward the main tank. Such check valves preventany air within the main tank 50 from being drawn back into the flexiblebladders 52 through the vent ports 80 during flight. However, duringrefueling, any gas in the flexible bladders 52 easily passes through thecheck valves from the flexible bladders 52 into the main tank 50.

Even after the transfer pump 60 is deactivated following emptying of theflexible bladders 52, the electronic flight controller 24 continues tooperate the in-tank pump/regulator 68 to deliver fuel 102 remaining inthe main tank 50 to the engine 26. In some arrangements, the main tank50 may even maintain positive pressure after the flexible bladders 52have been depleted of fuel 102 (e.g., if the venting pathways areprovisioned with check valves).

Ultimately, the main tank 50 eventually empties. During such emptying ofthe main tank 50, the negative pressure valve 66 allows air to enter themain tank 50 prevent the main tank 50 from encountering negativeinternal pressure that could cause fuel 102 to flow in the wrongdirection or cause fuel cavitation at the fuel pump inlet.

Once the aircraft 20 has completed its mission, the hybrid fuel system28 can be refueled by simply repeating the above-described process. Thatis, there is no need to evacuate the flexible bladders 52 beforerefueling. Rather, any gas in the flexible bladders 52 will be purgedthrough the vent ports 80 into the main tank 50. Accordingly, the hybridfuel system 28 is a rich and reliable fuel delivery mechanism, and iseasy to use and maintain. Further details will now be provided withreference to FIG. 4.

FIG. 4 shows a procedure 200 for operating a hybrid fuel system of anaircraft. At 202, a user prepares the hybrid fuel system of the aircraftto receive fuel from a fuel source, the hybrid fuel system including aset of flexible bladders in the aircraft wings and a main tank (also seeFIGS. 1-3). Along these lines, the user orients the set of flexiblebladders (e.g., by tilting the aircraft if necessary, by flexing thewings if necessary, etc.) so that the vent port of each flexible bladderis at the top. Additionally, the user opens a bypass valve between themain tank and the set of flexible bladders to provide a fuel path thatcircumvents a transfer pump which pumps fuel from the set of flexiblebladders into the main tank during flight. Furthermore, the userconnects the fuel source to the fill line leading to the main tank.

At 204, the user provides fuel from the fuel source into a fuelcontainment space defined by the set of flexible bladders and the maintank while venting gas out of the set of flexible bladders into the maintank. In particular, fuel enters the main tank first and then passesthrough the fuel path of the opened bypass valve into the set offlexible bladders. As fuel fills the flexible bladders (e.g., fillingthe sealed ends before filling the vented ends, also see FIG. 3), anygas within the set of flexible bladders is concurrently displaced intothe main tank through the vent ports. The user continues to provide fueluntil a positive pressure check valve releases the gas in the fuelcontainment space and then outputs fuel indicating that the fuelcontainment space is filled with fuel. The user then stops providingfuel from the fuel source into the main tank. The positive pressurewithin the fuel containment space (e.g., 2.5 psi) ensures that theflexible bladders have expanded into the available wing cavities forefficient use of space.

At 206, the user prepares the hybrid fuel system to deliver the fuel toan engine of the aircraft. Here, the user closes the bypass valve,removes the fuel source, and caps the fill line (also see FIG. 3). Theuser may then re-orient the aircraft if necessary in order for theaircraft to launch or lift off. At this point, the aircraft (e.g., aUAV) is properly fueled for flight.

As described above, improved techniques are directed to an aircrafthybrid fuel system 28 which includes a main tank 50 and a set offlexible bladders 52, each of which vents into the main tank 50. Withplacement of flexible bladders 52 in the aircraft wings 42, fuel 102 isable to be stored with less leakage risk than that of the conventionalwet-wing approach and more efficiently than that of the conventionalrigid tank approach. Additionally, fuel 102 within the flexible bladderscan be completely drawn down without concern over exposing suction portsof the flexible bladders 52 thus enabling full use of the bladder-storedfuel and freeing the aircraft of maneuvering restrictions. Furthermore,since each flexible bladder 52 vents to the main tank 50, there is noneed to evacuate the flexible bladders 52 to remove excess air duringrefueling. Rather, any gases within the flexible bladders 52 simply ventto the main tank 50 as the flexible bladders 52 are filled with fuel102. Moreover, both the main tank 50 and the set of flexible bladders 52can be fueled to a positive pressure thus maximizing the fuel storagecapacity of the hybrid fuel system 28.

While various embodiments of the present disclosure have beenparticularly shown and described, it will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present disclosure asdefined by the appended claims.

For example, it should be understood that the hybrid fuel system 28 wasdescribed above in the context of a UAV by way of example only. Othertypes are vehicles are suitable for use as well such as unmannedaircraft (UA) generally, organic air vehicles (OAVs), micro air vehicles(MAVs), unmanned ground vehicles (UGVs), unmanned water vehicles (UWVs),unmanned combat air vehicles (UCAVs), manned vehicles, and so on.

Additionally, it should be understood that the hybrid fuel system 28 wasdescribed above as having a branched fuel line 92 by way of exampleonly. In an alternative arrangement, separate fuel lines 92 lead fromthe fuel ports 82 to the input 94 of the transfer pump 60. In such andarrangement, each fuel line 92 may connect to the main tank 50 via arespective bypass valve 62.

Furthermore, it should be understood that the pump/regulator 68 thatdelivers fuel to the engine 26 was described above as residing withinthe main tank 50 by way of example only. In other arrangements, thepump/regulator 68 is a separate inline pump/regulator which is externalto the main tank 50. In some arrangements, the external pump/regulatorhas a return to tank line. Such modifications and enhancements areintended to belong to various embodiments of the disclosure.

What is claimed is:
 1. A method of operating a hybrid fuel system of anaircraft, the method comprising: preparing the hybrid fuel system of theaircraft to receive fuel from a fuel source, the hybrid fuel systemincluding a set of flexible bladders and a main tank; after the hybridfuel system of the aircraft is prepared to receive fuel from a fuelsource, providing fuel from the fuel source into a fuel containmentspace defined by the set of flexible bladders and the main tank whileventing gas out of the set of flexible bladders into the main tank; andafter fuel from the fuel source is provided into the fuel containmentspace, preparing the hybrid fuel system to deliver fuel to an engine ofthe aircraft; wherein each flexible bladder includes a venting end and asealed end; and wherein providing fuel from the fuel source into thefuel containment space while venting gas out of the set of flexiblebladders includes: orienting the aircraft at an angle in which theventing end of each flexible bladder is positioned above the sealed endof that flexible bladder, and after orienting the aircraft at the angle,supplying fuel to fill the sealed end of each flexible bladder prior tofilling the venting end of that flexible bladder.
 2. A method as inclaim 1 wherein each flexible bladder includes (i) a container portion,(ii) a fuel port coupled to the container portion, and (iii) a ventingport coupled to the container portion; and wherein supplying fuelincludes: providing fuel through the fuel port of each flexible bladderwhile gas within that flexible bladder exits that flexible bladderthrough the venting port of that flexible bladder.
 3. A method as inclaim 2 wherein the venting port of each flexible bladder connects tothe main tank through a respective venting tube; and wherein providingfuel includes: supplying fuel through the fuel port of each flexiblebladder while gas within that flexible bladder exits that flexiblebladder into the main tank through the venting port of that flexiblebladder and through the respective venting tube connecting that ventingport to the main tank.
 4. A method as in claim 3 wherein a transfer pumpis interconnected between the main tank and the set of flexiblebladders, the transfer pump being constructed and arranged to pump fuelfrom the set of flexible bladders into the main tank; and whereinsupplying fuel through the fuel port of each flexible bladder while gaswithin that flexible bladder exits that flexible bladder into the maintank includes: providing fuel into the set of flexible bladders througha set of paths that circumvents the transfer pump.
 5. A method as inclaim 4 wherein a bypass valve is interconnected between the main tankand the set of flexible bladders, the bypass valve being constructed andarranged to allow and disallow fuel to flow between the set of flexiblebladders into the main tank; and wherein supplying fuel through the fuelport of each flexible bladder while gas within that flexible bladderexits that flexible bladder into the main tank includes: transitioningthe bypass valve from a closed state to an open state to allow fuel toflow from the main tank into the set of flexible bladders, and after thebypass valve is transitioned to the open state, providing fuel from thefuel source into the main tank.
 6. A method as in claim 5 whereintransitioning the bypass valve from the closed state to the open stateincludes: forming a bypass valve path between the main tank and the setof flexible bladders, the bypass valve path running parallel to atransfer pump path between the main tank and the set of flexiblebladders, the transfer pump path being formed by the transfer pump, fuelin the main tank being allowed to flow from the main tank into the setof flexible bladders through the bypass valve path when the bypass valveis in the open state.
 7. A method as in claim 1 wherein the main tankincludes a hard tank; and wherein providing fuel from the fuel sourcefurther includes: filling the hard tank with fuel to sustain a positivefuel pressure within the hard tank.
 8. A method as in claim 7 wherein apositive pressure check valve is coupled to the hard tank; and whereinproviding fuel from the fuel source further includes: stopping fueldelivery to the hard tank when fuel escapes from the hard tank throughthe positive pressure check valve.
 9. A method as in claim 7 wherein anegative pressure check valve is coupled to the hard tank; and whereinthe method further comprises: freeing the negative pressure check valvefrom obstructions to enable air to pass through the negative pressurecheck valve into the hard tank to prevent occurrence of negativepressure in the hard tank.
 10. A method as in claim 1 wherein the set offlexible bladders includes a left wing bladder disposed in a left wingof the aircraft and a right wing bladder disposed in a right wing of theaircraft; and wherein supplying fuel to fill the sealed end of eachflexible bladder includes: expanding the left wing bladder and the rightwing bladder with fuel from the fuel source, the set of bladders beingconstructed and arranged to collapse as fuel is pumped from the set ofbladders into the main tank.
 11. A method as in claim 10, furthercomprising: operating a fuel delivery regulator to deliver fuel from themain tank to an engine of the aircraft and operating a transfer pumpwhich transfers fuel from the set of flexible bladders to the main tank.12. A method as in claim 11, further comprising: discontinuing operationof the transfer pump once fuel originally within the set of flexiblebladders has been transferred to the main tank while continuing tooperate the fuel delivery regulator to continue delivery of fuel fromthe main tank to the engine of the aircraft.
 13. A method as in claim 1wherein the aircraft is an unmanned aerial vehicle (UAV); wherein thefuel source is a portable fuel tank; and wherein providing fuel from thefuel source further includes: supplying fuel from the portable fuel tankinto the main tank through a fuel port of the UAV, the fuel filling theset of flexible bladders and venting gas from the set of flexiblebladders into the main tank as the fuel is supplied through the fuelport of the UAV.